What is KML?

Google Earth's format, and the reason it looks good - unlike GPX or GeoJSON it carries styling, so lines have colours and placemarks have icons. Still XML underneath, and still the thing to convert to when someone wants to see a route draped over terrain.

What is GPX?

The GPS exchange format: tracks, routes and waypoints, as XML. It's what watches, bike computers and handheld units produce, and what hiking and cycling apps import. Fixed fields, no room for extras, works everywhere.